1. Field of the Invention
The present invention relates to a novel electrolyte for a lithium secondary battery capable of providing a lithium secondary battery having superior battery cycle characteristics and battery characteristics such as electrical capacity, storage characteristics, and also relates to a lithium secondary battery using the same.
2. Description of the Related Art
In recent years, lithium secondary batteries have been widely used as the power sources for driving compact electronic devices etc. Lithium secondary batteries are mainly composed of a cathode, a non-aqueous electrolyte and an anode. In particular, a lithium secondary battery having a lithium complex oxide such as LiCoO2 as a cathode and a carbonaceous material or lithium metal as an anode is suitably used. Further, as the electrolyte for a lithium secondary battery, a cyclic carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is suitably used.
However, a secondary battery having an even more superior battery cycle characteristics and battery characteristics such as the electrical capacity is desired.
A lithium secondary battery using, as the cathode active material, for example, LiCoO2, LiMn2O4, LiNiO2, etc, suffers from a decrease in the battery performance since the solvent in the non-aqueous electrolyte is locally partially oxidized and decomposed at the time of charging and the decomposed products inhibit the desirable electrochemical reaction of the battery. This is believed to be due to the electrochemical oxidation of the solvent at the interface between the cathode active material and the non-aqueous electrolyte.
Further, a lithium secondary battery using a highly crystallized carbonaceous material such as natural graphite or artificial graphite, as an anode active material, sometimes suffers from peeling of the carbonaceous anode material which, depending on the extent of the phenomenon, makes the capacity irreversible. This peeling occurs due to the decomposition of the solvent in the electrolyte during charging, that is, is due to the electrochemical reduction of the solvent at the interface of the carbonaceous anode material and electrolyte. In particular, PC having a low melting point and high dielectric constant, has a high electroconductivity even at a low temperature. Nevertheless, when a graphite anode is used, there are problems that the PC cannot be used for the lithium secondary battery due to the decomposition thereof. Further, EC partially decomposes during the repeated charging and discharging thereof so that the battery performance is decreased. Therefore, the battery cycle characteristics and the battery characteristics such as the electrical capacity are not necessarily satisfactory at the present time.
The objects of the present invention are to solve the above-mentioned problems relating to an electrolyte for a lithium secondary battery and provide a non-aqueous electrolyte for a lithium secondary battery having superior battery cycle characteristics and battery characteristics such as electrical capacity and a lithium secondary battery using the same.
In accordance with the present invention, there is provided an electrolyte for a lithium secondary battery comprising (i) a non-aqueous solvent, (ii) an electrolyte salt dissolved therein and (iii) at least one alkyne derivative having the formulae (I), (II) and (III) or at least one alkyne carbonate derivative having the formula (IV):
R1xe2x80x94Cxe2x89xa1Cxe2x80x94R2xe2x80x83xe2x80x83(I) 
wherein R1, R2, R3 and R4 independently indicate a C1 to C12 alkyl group, a C3 to C6 cycloalkyl group, a aryl group, or hydrogen atom, R5, R6 and R7 in Y1, Y2, and Y3 independently indicate a C1 to C12 alkyl group, a C3 to C6 cycloalkyl group, or aryl group, and n is an integer of 1 or 2.
In accordance with the present invention, there is also provided a lithium secondary battery comprising (a) a cathode, (b) an anode, and (c) an electrolyte comprising (i) a non-aqueous solvent, (ii) an electrolyte salt dissolved therein and (iii) at least one alkyne derivative having the formulae (I), (II) and (III) or at least one alkyne carbonate derivative having the formula (IV):
R1xe2x80x94Cxe2x89xa1Cxe2x80x94R2xe2x80x83xe2x80x83(I) 
wherein R1, R2, R3 and R4 independently indicate a C1 to C12 alkyl group, C3 to C6 cycloalkyl group, aryl group, or hydrogen atom, R5, R6 and R7 in Y1, Y2, and Y3 independently indicate a C1 to C12 alkyl group, C3 to C6 cycloalkyl group, or aryl group, and n is an integer of 1 or 2.
The alkyne derivative contained in the electrolyte is reduced and decomposed, prior to the reduction and decomposition of the organic solvent, at the surface of the carbonaceous anode at the time of charging. A part of the decomposed product is believed to form a passivation film on the surface of the highly crystallized carbonaceous anode due to the activity of the natural graphite, artificial graphite, etc., whereby the reduction and decomposition of the organic solvent in the electrolyte are prevented in advance.
Further, a part of the decomposed product is believed to oxidize and decompose, prior to the reduction and decomposition of the organic solvent in the electrolyte at the slight excess voltage portion where the potential of the surface of the cathode material becomes high transitionally, whereby the oxidation and decomposition of the organic solvent in the electrolyte are prevented in advance.
Due to the above mechanism, it is believed there is the effect of suppressing decomposition of the electrolyte, without impairing normal reactions of the battery.
In the alkyne derivative contained in the electrolyte composed of the electrolyte salt dissolved in the non-aqueous solvent, R1, R2, R3, and R4 in the alkyne derivative of the formulae (I) and (II) are preferably, independently C1 to C12 alkyl groups such as a methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, octyl group and decyl group. The alkyl group may also be a branched alkyl group such as an isopropyl group or isobutyl group. Further, it may be a C3 to C6 cycloalkyl group such as a cyclopropyl group or cyclohexyl group. Further, it may include a C1 to C12 aryl group such as a phenyl group, benzyl group, or p-tolyl group. Further, a hydrogen atom is also possible.
Further, R5, R6, and R7 in Y1, Y2, and Y3 in the alkyne derivative of the general formulae (II) and (III) are preferably, independently from each other, a C1 to C12 alkyl group such as a methyl group, ethyl group, propyl group, butyl group, pentyl group, and hexyl group. Further, the alkyl group may be a branched alkyl group such as an isopropyl group or isobutyl group. Further, it may also be a C3 to C6 cycloalkyl group such as a cyclopropyl group or cyclohexyl group. Further, it may include a C1 to C12 aryl group such as a phenyl group, benzyl group, or p-tolyl group. However, n is an integer of 1 or 2.
The specific examples of the alkyne derivative having the formula (I) are 2-pentyne (R1=a methyl group, R2=a ethyl group), 1-hexyne (R1=hydrogen atom, R2=a butyl group), 2-hexyne (R1=methyl group, R2=propyl group), 3-hexyne (R1=R2=ethyl group), 1-heptyne (R1=hydrogen atom, R2=pentyl group), 1-octyne (R1=hydrogen atom, R2=hexyl group), 2-octyne (R1=methyl group, R2=pentyl group), 4-octyne (R1=R2=propyl group), 1-decyne (R1=hydrogen atom, R2=octyl group), 1-dodecyne (R1=hydrogen atom, R2=decyl group), phenylacetylene (R1=phenyl group, R2=hydrogen atom), 1-phenyl-1-propyne (R1=phenyl group, R2=methyl group), 1-phenyl-1-butyne (R1=phenyl group, R2=ethyl group), 1-phenyl-1-pentyne (R1=phenyl group, R2=propyl group), 1-phenyl-1-hexyne (R1=phenyl group, R2=butyl group), diphenylacetylene (R1=R2=phenyl group), 4-ethynyltoluene (R1=p-tolyl group, R2=hydrogen atom), dicyclohexylacetylene (R1=R2=cyclohexyl group), etc. However, the present invention is not limited to these compounds in any way.
Further, the specific examples of the alkyne derivative having the formula (II) are when Y1=xe2x80x94COOR5, 2-propynylmethyl carbonate (R3=hydrogen atom, R4=hydrogen atom, R5=methyl group, n=1), 2-propynylethyl carbonate (R3=hydrogen atom, R4=hydrogen atom, R5=ethyl group, n=1), 2-propynylpropyl carbonate (R3=hydrogen atom, R4=hydrogen atom, R5=propyl group, n=1), 2-propynylbutyl carbonate (R3=hydrogen atom, R4=hydrogen atom, R5=butyl group, n=1), 2-propynylphenyl carbonate (R3=hydrogen atom, R4=hydrogen atom, R5=phenyl group, n=1), 2-propynylcyclohexyl carbonate (R3=hydrogen atom, R4=hydrogen atom, R5=cyclohexyl group, n=1), 2-butynylmethyl carbonate (R3=methyl group, R4=hydrogen atom, R5=methyl group, n=1), 3-butynylmethyl carbonate (R3=hydrogen atom, R4=hydrogen atom, R5=methyl group, n=2), 2-pentynylmethyl carbonate (R3=ethyl group, R4=hydrogen group, R5=methyl group, n=1), 1-methyl-2-butynylmethyl carbonate (R3=methyl group, R4=methyl group, R5=methyl group, n=1), etc.
When Y1=xe2x80x94COR5, 2-propynyl acetate (R3=hydrogen atom, R4=hydrogen atom, R5=methyl group, n=1), 2-propynyl propionate (R3=hydrogen atom, R4=hydrogen atom, R5=ethyl group, n=1), 2-propynyl butyrate (R3=hydrogen atom, R4=hydrogen atom, R5=propyl group, n=1), 2-propynyl benzoate (R3=hydrogen atom, R4=hydrogen atom, R5=phenyl group, n=1), 2-propynylcyclohexyl carbonate (R3=hydrogen atom, R4=hydrogen atom, R5=cyclohexyl group, n=1), 2-butynyl acetate (R3=methyl group, R4=hydrogen atom, R5=methyl group, n=1), 3-butynyl acetate (R3=hydrogen atom, R4=hydrogen atom, R5=methyl group, n=2), 2-pentynyl acetate (R3=ethyl group, R4=hydrogen group, R5=methyl group, n=1), 1-methyl-2-butynyl acetate (R3=methyl group, R4=methyl group, R5=methyl group, n=1), etc.
When Y1=xe2x80x94So2R5, 2-propynyl methanesulfonate (R3=hydrogen atom, R4=hydrogen atom, R5=methyl group, n=1), 2-propynyl ethanesulfonate (R3=hydrogen atom, R4=hydrogen atom, R5=ethyl group, n=1), 2-propynyl propanesulfonate (R3=hydrogen atom, R4=hydrogen atom, R5=propyl group, n=1), 2-propynyl p-toluenesulfonate (R3=hydrogen atom, R4=hydrogen atom, R5=p-tolyl group, n=1), 2-propynyl cyclohexylsulfonate (R3=hydrogen atom, R4=hydrogen atom, R5=cyclohexyl group, n=1), 2-butynyl methanesulfonate (R3=methyl group, R4=hydrogen atom, R5=methyl group, n=1), 3-butynyl methanesulfonate (R3=hydrogen atom, R4=hydrogen atom, R5=methyl group, n=2), 2-pentynyl methanesulfonate (R3=ethyl group, R4=hydrogen group, R5=methyl group, n=1), 1-methyl-2-butynyl methanesulfonate (R3=methyl group, R4=methyl group, R5=methyl group, n=1), etc. However, the present invention is not limited to these compounds in any way.
Further, the specific examples of the alkyne derivative having general formula (III) are when Y2xe2x80x94COOR6 and/or Y3=xe2x80x94COOR7, 2-butyne-1,4-diol dimethyldicarbonate (R6=R7=methyl group, n is all 1), 2-butyne-1,4-diol diethyldicarbonate (R6=R7=ethyl group, n is all 1), 2-butyne-1,4-diol diphenyldicarbonate (R6=R7=phenyl group, n is all 1), 2-butyne-1,4-diol dicyclohexyldicarbonate (R6=R7=cyclohexyl group, n is all 1), etc.
When Y2xe2x80x94COR6 and/or Y3=xe2x80x94COR7, 2-butyne-1,4-diol diacetate (R6=R7=methyl group, n is all 1), 2-butyne-1,4-diol dipropionate (R6=R7=ethyl group, n is all 1), 2-butyne-1,4-diol dibenzoate (R6=R7=phenyl group, n is all 1), 2-butyne-1,4-diol dicyclohexanecarboxylate (R6=R7=cyclohexyl group, n is all 1), etc.
When Y2xe2x80x94SO2R6 and/or Y3=xe2x80x94SO2R7, 2-butyne-1,4-diol dimethanesulfonate (R6=R7=methyl group, n is all 1), 2-butyne-1,4-diol dipropanesulfonate (R6=R7=propyl group, n is all 1), 2-butyne-1,4-diol di-p-toluenesulfonate (R6=R7=p-tolyl group, n is all 1), 2-butyne-1,4-diol dicyclohexanesulfonate (R6=R7=cyclohexyl group, n is all 1), etc. However, the present invention is not limited to these compounds in any way.
In the alkyne derivatives, if the content of the alkyne derivative having the formulae (I), (II) and (III) is too large, the conductivity of the electrolyte etc. change and the battery performance is decreased in some cases. Further, if too small, a sufficient coating is not formed and the expected battery performance is not obtained. Therefore, the content is preferably in the range of 0.01 to 20% by weight, particularly 0.1 to 10% by weight, based upon the weight of the electrolyte.
The alkyne carbonate derivative contained in the electrolyte is reduced and decomposed, prior to the reduction and decomposition of the organic solvent in the electrolyte, at the surface of the carbonaceous anode at the time of charging. A part of the decomposed product is believed to form a passivation film on the surface of the highly crystallized carbonaceous anode due to the activity of the natural graphite, artificial graphite, etc., whereby the reduction and decomposition of the organic solvent in the electrolyte are prevented in advance.
Further, a part of the decomposed product is believed to oxidize and decompose, prior to the reduction and decomposition of the organic solvent in the electrolyte at the slight excess voltage portion where the potential of the surface of the cathode material becomes high transitionally, whereby the oxidation and decomposition of the organic solvent in the electrolyte are prevented in advance.
Due to the above mechanism, it is believed there is the effect of suppressing decomposition of the electrolyte, without impairing normal reactions of the battery.
In the alkyne carbonate derivative contained in the electrolyte composed of the electrolyte salt dissolved in the non-aqueous solvent, R1, R2, R3, and R4 in the alkyne carbonate derivative of the general formula (IV) are preferably, independently from each other, a C1 to C12 alkyl group such as a methyl group, ethyl group, propyl group, butyl group, pentyl group, and hexyl group. The alkyl group may also be a branched alkyl group such as an isopropyl group or isobutyl group. Further, it may be a C3 to C6 cycloalkyl group such as a cyclopropyl group or cyclohexyl group. Further, it may include a C1 to C12 aryl group such as a phenyl group, benzyl group, or p-tolyl group. Further, a hydrogen atom is also possible and n is an integer of 1 or 2.
The specific examples of the alkyne carbonate derivative having the general formula (IV) are dipropargyl carbonate (R1=R2=R3=R4=hydrogen atom, n=1), di(2-butynyl)carbonate (R1=R4=methyl group, R2=R3=hydrogen atom, n=1), di(3-butynyl)carbonate (R1=R2=R3=R4=hydrogen atom, n=2), di(2-pentynyl)carbonate (R1=R4=ethyl group, R2=R3=hydrogen atom, n=1), di(1-methyl-2-butynyl)carbonate (R1=R4=methyl group, R2=R3=methyl group, n=1), 2-propynyl-2xe2x80x2-butynyl carbonate (R1=R2=R3=hydrogen atom, R4=methyl group, n=1), etc. However, the present invention is not limited to these compounds in any way.
In the alkyne carbonate derivatives, if the content of the alkyne carbonate derivative having the formula (IV) is too large, the conductivity of the electrolyte etc. change and the battery performance is decreased in some cases. Further, if too small, a sufficient coating is not formed and the expected battery performance is not obtained. Therefore, the content is preferably in the range of 0.01 to 20% by weight, particularly 0.1 to 10% by weight, based upon the weight of the electrolyte.
The non-aqueous solvent used in the present invention preferably comprises a mixture of a high dielectric solvent and a low viscosity solvent.
Examples of the high dielectric solvent are preferably a cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). These high dielectric solvents may be used alone or in any combination thereof.
Examples of the low viscosity solvent are a linear carbonate such as dimethyl carbonate (DMC), methylethyl carbonate (MEC), and diethyl carbonate (DEC), methylpropyl carbonate (MPC), butylmethyl carbonate (BMC), methylisopropyl carbonate (MIPC), isobutylmethyl carbonate (IBMC), sec-butylmethyl carbonate (SBMC) and tert-butylmethyl carbonate (TBMC), an ether such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane, a lactone such as xcex3-butyrolactone, xcex3-valerolactone, a nitrile such as acetonitrile, an ester such as methyl propionate, and an amide such as dimethyl formamide. These low viscosity solvents may be used alone or in any combination thereof.
The high dielectric solvent and low viscosity solvent are freely selected and combined for use. Note that the high dielectric solvent and low viscosity solvent are usually used in a ratio by volume (high dielectric solvent:low viscosity solvent) of 1:9 to 4:1, preferably 1:4 to 7:3.
Examples of the electrolyte salt used in the present invention are LiPF6, LiBF4, LiClO4, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2CF3)3, LiPF4(CF3)2, LiPF3(C2F5)3, LiPF3 (CF3)3, LiPF4 (iso-C3F7)2, LiPF5(iso-C3F7), etc. These salts may be used alone or in any combination thereof. These salts are normally dissolved in the non-aqueous solvent in a concentration of 0.1 to 3M, preferably 0.5 to 1.5M.
The non-aqueous electrolyte of the present invention can be obtained by, for example, mixing the high dielectric solvent or low viscosity solvent, dissolving the electrolyte salt therein, and dissolving at least one alkyne derivative having the formulae (I), (II) and (III), or at least one alkyne carbonate derivative having the formula (IV).
The electrolytes of the present invention can be used as a component member of a secondary battery, in particularly as a component member of a lithium secondary battery. The other component members of the secondary battery are not particularly limited. The various component members conventionally used in the past may be used.
For example, as the cathode active material, a complex metal oxide of at least one metal selected from the group consisting of cobalt, manganese, nickel, chrome, iron, and vanadium with lithium is used. Examples of such a complex metal oxide are LiCoO2, LiMn2O4, LiNiO2, etc.
The cathode is prepared by mixing the cathode active material with a conductive agent such as acetylene black or carbon black and a binder such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) to make a cathode paste, then rolling this cathode paste on a collector such as aluminum foil or a stainless steel foil or lathe, then heat treating at a temperature of 50 to 250xc2x0 C. for about 2 hours in vacuum.
As the anode (or anode active material), lithium metal or a lithium alloy and a carbonaceous material (heat cracked carbons, coke, artificial graphite, natural graphite, an organic polymer compound sintered product, carbon fiber) having a graphite-type crystal structure able to absorb and discharge lithium, a complex tin oxide, etc. may be used. In particular, a carbonaceous material having a graphite-type crystal structure having a lattice spacing (d002) of the lattice face (002) of 0.335 to 0.340 nm (i.e., nanometer) is preferably used. Note that the powder material such as the carbonaceous material is mixed with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF) to make an anode paste.
The structure of the lithium secondary battery is not particularly limited. A coin battery having a cathode, anode, single layer or multiple layer separator and further, a cylindrical battery, prismatic battery, etc. having a cathode, anode, and roll-shaped separator may be mentioned as examples. Note that as the separator, a known polyolefin porous film, woven fabric, nonwoven fabric, etc. is used.